It’s not like they can’t steal a helmet when they steal the bike.
This helmet culture is pathetic.
Even in this insane safety-NAZI society, you will almost never find a bike sitting somewhere in public, casually tossed on the side, with a helmet attached.
So yes, they could do so, but usually that would be stealing from a deadly serious “cyclist” (why are bicycles riders “cyclists” but motorcycles “bikers”?) spandex highway-rider who thinks his bike is the equal of a car. They’re about the only ones that MIGHT have a helmet on their “parked” bike - but then, they wear them on their heads almost constantly.